This invention relates in general to a frequency discriminator for use in a doppler radar unit having a moving and stationary mode of operation. In particular, this invention relates to a method and apparatus for separating the received doppler signal into its platform speed and target speed frequency components by means of frequency translation techniques.
The use of doppler radar techniques for measuring the speed of moving objects and more particularly for measuring the speed of motor vehicles is well known in the art. In these systems, an ultra high frequency radar signal is radiated toward the vehicle under observation. Upon contact with the target vehicle, a portion of the transmitted doppler wave is reflected back to the radar unit. Thereafter, the reflected wave is received and mixed with a sample portion of the transmitted wave to measure the difference in frequency between the reflected signal and the transmitted wave. This frequency differential is produced by the relative motion of the target vehicle with respect to the measuring device and is proportional to the ground speed of the target vehicle. By their nature, raw or unfiltered doppler return signals are often very inaccurate. This inaccuracy is due to a large noise level that typically occurs because of white noise, reflection difficulties due to multiple targets and/or the dropping of pulses at critical intervals so that incorrect and erroneous displays are prevalent in conventional prior art devices. A general solution to these prior art difficulties is given and described in U.S. patent to Berry U.S. Pat. No. 3,689,921, issued Feb. 5, 1972, which is incorporated by reference herein.
It has become apparent that the efficiency of selective electronic speed limit law enforcement can be greatly enhanced if the radar platform, conventionally a police patrol car, is capable of movement during such enforcement and surveillance. A number of devices presently known in the art attempt to impart movement capability to the radar platform vehicle. An example of such a radar device is described in the U.S. patent Aker et al., U.S. Pat. No. 3,936,824, issued Feb. 3, 1976, which is incorporated by reference herein.
The Mahler U.S. Pat. No. 3,394,371, issued July 23, 1968, and titled Vehicle Motion Nulling System is believed to represent a complex missle tracking circuit which employs doppler radar and some translation techniques.
When both the target vehicle and the radar platform vehicle are in motion, the received doppler signal is a complex wave composed of at least two frequency components. One of the received frequency components is produced by the waves reflected from the road and other stationary objects in the environment and is related to the ground speed of the platform vehicle. The second frequency component is generated by the waves reflected from the target vehicle and is representative of the relative speed of the target vehicle with respect to the radar platform vehicle. This frequency component is related to the sum speed of the two vehicles if they are moving in opposite directions or to the difference in speed between the two vehicles if they are moving in the same direction. It should be noted that the frequency of the relative speed component will always be higher than the frequency of the platform speed frequency component if the radar platform vehicle and the target vehicle are traveling in opposite directions.
The ground speed of an approaching target vehicle is typically computed by first obtaining the composite frequencies of the received doppler signals and then separating the received signals into platform and sum speed frequency components. These frequency components are then correlated with a time base to obtain digital numbers representative of the platform speed and the sum speed of the two vehicles. Thereafter, the computed platform speed is subtracted from the sum speed leaving a digital number representative of the ground speed of the target vehicle. As illustrated above, the proper computation of the resultant target speed is dependent upon the effective separation of the received doppler signal into its respective frequency components. Prior art radar devices perform the required frequency discrimination by means of selective filtering systems that employ tracking filters which are tunable over the entire range of doppler input frequencies. The center frequency of these tracking filters is initially set to the speed of the radar platform vehicle and then continuously updated during the detection cycle. The tracking function performed by these systems necessitates the use of very complicated and expensive circuitry.
My invention, on the other hand, uses frequency translation techniques and preset filters to accomplish the required frequency discrimination. The platform speed and sum speed frequency components are frequency translated in the present invention in such a way as to lock the translated platform speed frequency component to a preselected reference frequency. The required frequency discrimination is then performed by a narrow band pass filter and a wide band pass filter. The narrow band pass filter has a center frequency which is equal to the preselected reference frequency and is therefore operable to pass only the frequency translated platform speed frequency component. The wide band pass filter, on the other hand, allows passage of the frequency translated sum speed frequency component while supressing the translated platform speed frequency component. In this way, the required frequency discrimination is performed by filters which are fixed in frequency and properly selected so that only one frequency component is passable by each of the filters. The separated frequency components are then frequency translated downward to provide a platform speed frequency signal and a target speed frequency signal. The frequency of the platform speed frequency signal is related to the ground speed of the platform vehicle while the frequency of the target speed frequency signal is related to the ground speed of the target vehicle allowing direct computation of target speed.
It has been found to be of benefit in the application of doppler radar for measurement of vehicular speeds to aurally monitor the target doppler signal, especially in multiple target situations. Accordingly, several manufacturers presently incorporate audio circuitry in their radar units to accomplish this. In all prior art devices, the audio note heard in the moving mode of operation is representative of the closing rate between the platform vehicle and the target vehicle, rather than the rate of the target vehicle alone.
In the present invention, the frequency translation results in a subtraction of the platform doppler frequency component from the combined speed frequency component, leaving a target speed frequency component which is independent of platform velocity. The frequency or doppler note heard by the operator is therefore a function only of target speed and does not vary with platform as is the case in other radar units.
The frequency translation technique employed in the present invention is accomplished by using the received doppler signal to modulate a carrier signal which is generated by the voltage controlled oscillator (VCO) of a phase locked loop. The phase locked loop is operable to compare the frequency and phase of the frequency translated platform speed frequency component with the preselected reference signal and to produce a carrier signal capable of keeping these two signals locked in frequency and phase. After frequency discrimination, the carrier signal is used to demodulate the reference frequency signal to provide the platform speed frequency signal. The target speed frequency signal, on the other hand, is obtained by using the reference signal to demodulate the frequency translated sum speed frequency component.
A lock detection circuit and a search sweep circuit are also provided with the frequency discriminator of this invention. If the frequency translated target speed frequency component is not equal to the frequency of the reference signal, then the lock detection circuit will inhibit the output display and activate the search sweep circuit. The search sweep circuit provides an auxillary control to the phase locked loop causing the VCO of the phase locked loop to sweep over its entire frequency range until the frequency translated platform speed frequency component approaches the reference frequency. When these two frequencies come sufficiently close to each other, the lock detection circuit inhibits the sweep operation and allows the phase locked loop to lock onto the frequency translated platform speed frequency component. Once the phase locked loop has locked onto this frequency, the detected output of the loop will vary in response to deviations in the platform speed and will adjust the VCO to maintain the lock status between these two signals. The inhibit signal is also deactivated and the output signals are once again provided for display.
It should be pointed out at this time that the frequency discrimination techniques discussed herein can also be used to separate the platform and target speed frequency components when both vehicles are moving in the same direction. When both vehicles are moving in the same direction, separation of the platform and target speed frequency components can be performed by a third band pass filter which has an operable frequency range below the reference frequency.
Even though the use of modulators for frequency translation is well known in the communications field, it has never to my knowledge been applied in the doppler radar art to separate the received doppler signal into its respective frequency components. While it is possible to directly obtain the target speed frequency component by demodulation techniques, this method of frequency discrimination is highly undesirable because prefiltering the platform and target speed frequency component is required and products of this discrimination technique tend to fall within the band of the platform and target frequencies creating spurious frequency signals. Frequency translation, on the other hand, eliminates these spurious frequencies and allows simple filtering after translation.
In a second embodiment of the present invention, the narrow band filter is replaced by a linear phase detector which is only responsive to frequency signals that are within a finite phase of the reference frequency. In order to assure that the frequency translated platform speed frequency component is locked to the reference frequency, a second linear phase detector called the lock detector is provided to compare the phase of the frequency translated platform speed frequency component with the reference signal 90.degree. out of phase. The output of the lock detector is then sent to a lock detection circuit which determines whether or not the frequency translated platform speed frequency component is properly locked to the reference frequency. In this embodiment of my invention, a second phase locked loop, which is locked to the frequency translated platform speed frequency component, is provided to smooth the reference carrier signal produced in the first phase locked loop. The output of the second phase locked loop is then used to obtain the platform speed and target speed frequency signals. Additional lock detection circuits are also provided to assure that all phases of the separation procedure are properly locked.
It is accordingly an object of my invention to provide a doppler radar unit that uses a unique method and apparatus for detecting the speed of an approaching target vehicle when the radar platform vehicle is stationary or moving.
A further object of my invention is to provide a doppler radar unit that produces a speed readout indicative of the relative speed of the target vehicle when both the radar platform and target vehicles are in motion without the need for any mechanical linkage with the vehicle acting as the radar platform.
A further object of my invention is to provide a doppler radar unit for detecting the speed of a target vehicle that uses a simple method for separating the received doppler signal into its respective frequency components representative of the ground speed of the platform vehicle and the target vehicle.
Another object of my invention is to provide a doppler radar unit for detecting the speed of a target vehicle that does not require a great amount of complicated electrical circuitry to track the speed of the radar platform vehicle and to tune the tracking filters over the entire frequency range of interest.
A further object of my invention is to provide a doppler radar unit for detecting the speed of a target vehicle wherein the received doppler signal is separated into its various frequency components representative of the speed of the radar platform vehicle and the target vehicle by means of frequency translation techniques and fixed frequency filters.
Another object of my invention is to provide a doppler radar unit for detecting the speed of an approaching target vehicle wherein the received doppler signal is separated into its respective frequency components representative of the ground speed of the platform vehicle and the target vehicle by first frequency translating the received doppler signal upward to a preselected reference frequency, then filtering the translated signal to separate it into its speed components, and finally frequency translating the signal downward to produce speed signals representative of the speed of the radar platform vehicle and the target vehicle.
A further object of my invention is to provide a doppler radar with a monitoring system which will allow an audio signal to be heard that is a function of the target speed only and is not a function of the combined speed between the platform vehicle and the target vehicle.
A further object of my invention is to provide a doppler radar unit for detecting the speed of a target vehicle that includes a valid signal detection circuit operable to inhibit velocity display if the frequency translated platform speed frequency component is not properly locked to the preselected reference frequency.
Another object of my invention is to provide a doppler radar unit for detecting the speed of a target vehicle that includes a search sweep circuit operable to search the entire frequency of interest to lock the frequency translated platform speed frquency component to the preselected reference frequency.
A further object of my invention is to provide a doppler radar unit for detecting the speed of a target vehicle that is capable of smoothing the received doppler signal to improve the accuracy of the velocity computation.
A further object of my invention is to provide a doppler radar unit for detecting the speed of a target vehicle that requires only one antenna system.
Other and further objects of this invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.